In recent years, the miniaturization and high densification of hard disk drives (HDD) has rapidly advanced and further advances in densification are expected. High densification of an HDD is possible by increasing track density through decreasing memory track width. However, when track width is decreased, the size of the recorded magnetization, specifically the recorded signal, decreases, and this leads to the necessity of an enhancement in the reproduction sensitivity of the magneto-resistive (MR) head, which reproduces the media signals.
Recently, tunneling magneto-resistance (TMR) heads including highly-sensitive spin-valve films using the tunneling magneto-resistance effect are being used. A spin-valve film is a stacked film with a sandwich structure including a spacer layer between two ferromagnetic layers. A magnetization direction of a first of the two ferromagnetic layers (referred to as a “pinned layer” or a “magnetization fixing layer”) is fixed by an antiferromagnetic layer or the like. A magnetization direction of a second of the ferromagnetic layers (referred to as a “free layer” or a “magnetization free layer”) is modifiable by an external magnetic field. A great amount of magneto-resistance effect can be obtained by varying the relative angles of the magnetization directions of the two ferromagnetic layers in the spin-valve film. Here, the sandwich structure where the spacer layer is sandwiched between the two ferromagnetic layers is referred to as a “spin-dependent resistance variation unit”.
Examples of magneto-resistive effect devices using spin-valve films include CIP (Current In Plane)-GMR devices, CPP (Current Perpendicular to Plane)-GMR devices, and TMR (Tunneling Magneto-Resistance) devices. In CIP-GMR devices, sense current flows parallel with the spin-valve film face, and in CPP-GMR devices and TMR devices sense current flows substantially perpendicular to the spin-valve film face.
In a system in which the sense current flows in a direction perpendicular to the film face, a metal layer is used as the spacer layer in regular CPP-GMR devices, and an insulating layer is used as the spacer layer in TMR devices.
In order to enhance the memory density of a hard disk drive and also to enhance the data transmission rate, excellent high-frequency responsiveness of the reproduction head is required. Therefore, it is necessary to reduce the resistance of the reproduction head device. Reducing the thickness of a tunneling insulating layer is effective for reducing the resistance of conventional TMR devices. However, if the thickness of the tunneling insulating layer is reduced excessively, a large number of pin holes will form in the insulating layer. This leads to problems related to the deterioration of the characteristics of the TMR device such as reductions in the MR rate of change and/or increases in noise due to an increase in magnetic coupling of the free layer and the pinned layer.
On the other hand, in CPP-GMR devices a metal layer is used as the spacer layer. Therefore, from the perspective of reducing resistance, CPP-GMR devices are more advantageous than TMR devices. However, there is a problem in that with CPP-GMR devices, a sufficiently large MR rate of change cannot be obtained.
In order to solve the problems described above, efforts are being made such as modifying the structure of CPP-GMR devices, selecting the material of the spacer layer, and the like.
For example, a CPP device constituted by a nano-oxide layer (NOL) including a current path in a thickness direction, instead of a simple metal layer, as the spacer layer used in a CPP-GMR device has been proposed. With this device, the MR rate of change can be increased due to a current-confined-path effect. Such a device is referred to as a “CCP-CPP device”.
Aside from the effort described above, magneto-resistive effect devices have been proposed in which a thin film spin filter (SF) layer formed from oxides or nitrides is inserted in the ferromagnetic layers and/or at an interface between these and a nonmagnetic spacer layer. This SF layer can increase the MR rate of change due to having a spin filtering effect by which the flow of either up spin electrons or down spin electrons is blocked.